A battery separator is used to separate the battery's positive and negative electrodes, and is typically microporous so that ions may pass therethrough to the positive and negative electrodes. In lead/acid storage batteries, either automotive or industrial batteries, the battery separator is typically a microporous polyethylene separator having a back web and a plurality of ribs standing on the back web. See: Besenhard, J. O., Editor, Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany (1999), Chapter 9, pp. 245–292. The separators for automotive batteries are typically made in continuous lengths and rolled, subsequently folded, and sealed along its edges to form pouches that receive the electrodes for the batteries. The separators for industrial (traction) batteries are typically cut to a size about the same as an electrode plate.
U.S. Pat. No. 3,917,772 illustrates a method of making a lead/acid battery separator sheet from a plastic material. In this method, the sheet is calender molded to form ribs and/or projections. Referring to U.S. Pat. No. 3,917,772's FIGS. 2 and 3, ribs 29 and 31 have a solid profile. Referring to U.S. Pat. No. 3,917,772's FIGS. 4 and 5, discrete projections 45 and 49 are formed by rounded pits in the calendering rolls 12 and 13. U.S. Pat. No. 4,000,352 illustrates a lead/acid battery separator characterized by an interrupted pattern of discrete separator projections standing on a back web. Each projection has a circular or oval shape. U.S. Pat. No. 5,558,952 illustrates a lead/acid battery separator having a plurality of discrete ribs with no intermediate connecting walls. U.S. Pat. No. 5,716,734 illustrates a lead/acid battery separator having a plurality of ribs, each with a solid profile. PCT Publication WO 01/13442 illustrates a lead/acid battery separator having at least one vertical rib and a plurality of studs. The studs are truncated cones and their bases are flush with the back web. The ribs have a solid profile.
While the foregoing prior art, that is those having the projections, studs, or discontinuous ribs, have advanced the art, they are deficient because of their inability to prevent the formation of gas pockets. In a typical lead/acid battery separator, the separator has a ribbed face (i.e., with the primary ribs) and a back face (i.e., without ribs or a plurality of small or secondary ribs). The negative electrode (plate) is placed adjacent to the back face, and the positive electrode (plate) rests on the ribs of the ribbed face. Once a battery is sufficiently charged and current is continually applied (i.e., overcharging), hydrogen is generated at the negative plate, and oxygen is generated at the positive plate. As hydrogen is formed at the negative plate, it may push the separator away from the negative plate thereby forming a gas channel or pocket. A channel allows the hydrogen gas to escape, which is good; a pocket, on the other hand, prevents the escape of gas, which is bad because the pocket becomes an area of infinite resistance in the battery. Furthermore, if the separator's back web is pushed to contact the positive plate, the back web can oxidize and a hole can form.
Accordingly, there is a need to improve these battery separators.